This invention relates to improved methods for initiating the decomposition of hydrazine fuels.
Hydrazine-based fuels are widely used as energy sources in monopropellant rocket engine and gas generator systems. These systems operate by introducing the hydrazine component into a reaction chamber, wherein it is caused to undergo an exothermic decomposition into gaseous products. An important consideration in such systems is the choice of means for initiating and sustaining the decomposition. Iridium has proven to be an effective catalyst for hydrazine decomposition, but it is in limited supply and quite expensive. Other elements capable of acting as catalysts include iron, nickel, cobalt, ruthenium and molybdenum. However, none of these other elements react spontaneously with hydrazine fuels at ambient temperature, and they therefore require some auxiliary means of initiating the decomposition reaction.
Three types of auxiliary initiation means have been described: pyrotechnic squibs, liquid hypergolic initiators, and solid hypergolic initiators. The use of pyrotechnic squibs limits the number of restarts to the number of squibs carried. Furthermore, squibs are susceptible to premature initiation by radio frequency induction and stray currents. The use of liquid hypergolic initiators detracts from the inherent reliability and simplicity of monopropellant systems by requiring a dual set of tanks, filters, valves and injectors. Solid hypergolic initiators have been tested on various occasions, but so far none have offered the necessary reactivity and environmental stability required for flight applications.
To be effective, a solid initiator must produce spontaneous ignition upon contact with hydrazine fuels. For many applications (e.g. aircraft), it must also maintain such reactivity down to about -65.degree. F. To date, the most widely used solid initiator has been iodine pentoxide. This compound is a powder at ambient temperature, but it sublimes readily at temperatures above 575.degree. F. It is very hygroscopic and deliquescent and, in the presence of even minute amounts of moisture, eventually converts to a syrup-like corrosive liquid. When iodine pentoxide is used, it is imperative to encapsulate it hermetically in the reactor to prevent it from migrating by sublimation and to avoid absorption of water.